If you are receiving this error from iTunes when trying to authorize and play an Audible book, Could not authorize this computer because either your user name or password are invalid, even when you know that you are entering your credentials 100% correctly, it may be a problem with your password.

Audible is now owned by Amazon, meaning that you can use your Amazon username and password to login. Amazon allows for very secure passwords, such as asd;kj@!#$asldasdl!!@3123jc. However, when you have a password that uses special characters, iTunes seems to reject them saying they are invalid. The solution to this is simple: change your password to something that is only alpha-numeric.

This issue seems to be a carry-over from the old Audible account logins, as can be seen in this support document.